Sen. Bernie Sanders (I-Vt.) has attracted a passionate following because he is selling his followers a fantasy. And not just any fantasy — but one of epic proportions. A group of respected, nonpartisan experts offered the public a sense of the scale on Monday, releasing the most thorough analysis yet on Sanders’s plan and finding that it is profoundly flawed. Before the Democrats agree to adopt his agenda, a price they might pay to get him to end his doomed campaign, they should be clear about what they are signing onto.

Democratic presidential candidate Bernie Sanders rallied supporters in New Jersey on May 8. He promised to continue campaigning until the last primary, and reiterated that the most important things is that "Donald Trump must not become president." (Reuters)

The analysts found that Sanders’s program is very progressive, jacking up taxes massively on the rich and providing a range of new benefits to the poor. Though every working person would face higher payroll taxes, the value of these benefits would leave 95 percent of households better off. They granted Sanders that his single-payer health system, in which the government would pay for everyone’s care and demand no co-payments or other cost-sharing, would expand health-care access and cut the growth of health-care costs, and they made “aggressive” assumptions about how much the new system would reduce drug and other prices.

But there is a massive catch. Sanders’s assurance that he has “a plan to pay for every spending program he has introduced to date” is wrong. And not just wrong, but extravagantly so. Even with his large tax increases, Sanders would fall $18 trillion short over just 10 years. Factoring in interest costs, his plan would add $21 trillion to the debt over a decade. That is more than the roughly $19 trillion the Treasury already owes. And the picture would probably get much worse as time went on. Expanding Social Security, for example, would become much more expensive as more people retired.

Sanders’s health-care plan is the big budget-buster. It alone would cost $32 trillion over 10 years — “more than twice the new revenues” that Sanders would raise, Len Burman, the director of the Urban-Brookings Tax Policy Center, said on Monday. When reporters have asked Sanders about the costs of his health-care plan in the past, the senator has typically responded that other countries maintain functioning single-payer systems — as though he has not proposed a specific plan that should be assessed on its own merits. Burman pointed out Monday that other countries do not promise their people as much as Sanders does. Making his system less costly, the analysts found, would require doing very unpopular things. Deeply cutting payments to doctors and hospitals, for example, would induce “supply constraints” — that is, a shortage of providers willing to meet demand for health services, which would make it harder to access care. Think long wait times and other hurdles.

Fully funding Sanders’s spending plan, meanwhile, would be extremely difficult. He has already proposed raising taxes on the wealthy so dramatically that tapping upper incomes much more would start losing the federal government revenue, Burman explained. Instead, Sanders would have to significantly increase broad-based taxes, such as on consumption, to fill the gap.

The last option is simply letting the national debt balloon. Burman warned that conventional analysis would show this strategy “hurting the economy, potentially significantly.”

Sanders articulates broad principles — free health care for all; free college for everyone. When pressed on the details, he falls back on broad explanations — other countries to seem to do these sorts of things, so we can, too. But in the real world of policymaking, where specific numbers must add up and tradeoffs must be considered, his program is a flop.

Sanders is not thinking big, as he often puts it. He is thinking carelessly. Those attracted to his message of expanding health-care access and shoring up the safety net, which are worthy goals, should know that he does not have a real plan to do these things. Instead, Sanders offers unchecked ideological certainty and unkeepable promises.

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.

Stephen StrombergStephen Stromberg is a Post editorial writer. He specializes in U.S. policy and politics, covering elections, the White House, Congress, legal affairs, energy, the environment and health care. Stromberg rejoined The Post's opinions section in 2009 as deputy opinions editor for digital before shifting back into full-time editorial writing. Follow